Thiourea finds application in manufacture of amino resins, herbicides, fungicides, insecticides, plant growth regulators, photographic paper. It finds use in electrochemical processes, pharmaceutical industries, textile processing, hydrometallurgy, rubber industry and petroleum industry.
In the prior art thiourea is presently produced essentially by the following methods.
(a) from ammonium sulphocyanide by isomerisation in molten condition:
(b) from Calcium Cyanamide and ammonium sulphide:
(c) from Calcium Cyanamide and calcium hydrosulphide:
(d) from Calcium Cyanamide and hydrogen sulphide and subsequent conversion by method (c).
The above mentioned processes are briefly described herein-below.
(a) The isomerisation of ammonium sulphocyanide involves melting the ammonium thiocyanate and keeping upto temperature level of 140-170xc2x0 and dissolving the thiocarbamide from the cooled ground melt.
NH4SCNxe2x86x92CS(NH2)2 
(b) The preparation by the reaction between calcium cyanamide and ammonium sulphide involves addition of the solution of the latter at 25-80xc2x0 C. and blowing off ammonia, or in the presence of an ammonium salt such as carbonate, sulphate or oxalate.
(c) The calcium hydrosulphide method involves addition of cyanamide to a solution of calcium hydrosulphide such that the temperature is controlled around 70-80xc2x0 C. The reaction is:
2CaCN2+Ca(SH)2+6H2Oxe2x86x923Ca(OH)2+2CS(NH2)2 
The product is isolated from the reaction mass by filtration, concentration at temperatures below 98xc2x0, filtration to remove calcium salts and crystallisation.
(d) The method using reaction between calcium cyanamide and hydrogen sulphide consists of passing a current of hydrogen sulphide in a suspension of calcium cyanamide with good agitation and avoiding undue rise in temperature.
The reaction is:
CaCN2+3H2Sxe2x86x92Ca(SH)2+CS(NH2)2 
The Ca(SH)2 is converted into thiourea by further addition of calcium cyanamide according to the following reaction.
2CaCN2+Ca(SH)2+6H2Oxe2x86x922CS(NH2)2+3Ca(OH)2 
Besides the above the other methods include the following.
(a) Treatment of CaCN2 with H2S in the presence of small amount of water, methanol, ethylacetate, aniline or esters, hydrocarbons like benzene or their halogen derivatives.
(b) Reacting together CaCN2 and alkaline earth sulphide in presence of water and carbon dioxide in a ball mill.
(c) Reacting alkaline earth cyanamide such as CaCN2 with SO2 or CO2 till calcium is removed and making alkaline with ammonia and further reaction with H2S and completion of the reaction by heating.
(d) Converting CaCN2 by hydrolysis to Ca(HCN2)2 and passing H2S and subsequent treatment to remove calcium by CO2 containing gases.
(e) Treating CaCN2 in water with sulphuric acid at low temperatures, making ammoniacal and treatment with H2S at low temperature and acidifying and cooling.
(f) By reaction between cyanamide and Na2S in aqueous phosphate or borate buffers at pH 6-10 at 25xc2x0 C.
(g) By simultaneous action of CO2 and H2S in high H2S ratios on aqueous suspension of calcium cyanamide.
(h) Reaction of As2S3 ore in presence of sulphur with CaCN2 such that the ratio of Nitrogen and S are in the ratio 1:1.
The above methods suffer from the following draw-backs:
1. The isomerisation methods given conversions only upto a maximum of 25% at the most and isolation involves tedious methods.
2. Ammonium sulphide is not commercially available and the method involving its use produces ammonia. Impurities like sulphate are also present.
3. The method using calcium hydrosulphide or hydrogen sulphide produces lime and its removal completely is not possible. Impurities like calcium trithiocarbonate and calcium sulphocyanide are formed. This requires discarding of mother liquors where thiourea is lost. Formation of calcium hydroxide is detrimental to the product.
4. The reaction between calcium cyanamide and hydrogen sulphide suffers from the following counts:
a) The consumption of hydrogen sulphide is high and produces equimolar quantity of calcium hydrosulphide.
b) The calcium hydrosulphide on reaction with calcium cyanamide produces calcium hydroxide which is difficult to remove as stated in (3) above.
5. The process which use simultaneous use of CO2 and H2S consume a large amount of expensive and toxic H2S and no recovery of the unused H2S is indicated. The auto-genous temperature rise may give rise to sulphur components other than thiocarbamide.
6. Methods involving use of As2S3 and Na2S are of limited use and applications due to very stringent reaction conditions.
The object of the present invention is to provide an improved process for the preparation of Thiourea using Calcium Cyanamide, carbon dioxide and hydrogen sulphide.
Accordingly the present invention provides an improved process for the preparation of thiourea which comprises passing a mixture of carbon dioxide and hydrogen sulphide into a slurry formed by addition of major part of calcium cyanamide charge into water under constant stirring, maintaining alkaline pH at a temperature ranging between ambient to 80xc2x0 C., stopping the addition of hydrogen sulphide, continuing the slow passing of carbon dioxide and addition of remaining part of calcium cyanamide charge and retaining the reaction mass to complete the secondary reactions to form the product for a period ranging from 2 to 5 hours and continuing passing of carbon dioxide at an increased rate for effecting decomposition of Ca(SH)2 for a period of 1.6 to 6 hours, stopping the addition of carbon dioxide, separating thiourea solution by conventional methods, treating the separated solution with activated carbon, removing the carbon by conventional methods, separating the product formed by conventional methods and drying the product at a temperature between 50 to 70xc2x0 C. to obtain the product.
In an embodiment of the present invention the pH of the reaction mixture maintained may be at a value 8.0 to 11.0.
In another embodiment of the present invention the rate of addition of carbon dioxide may range between 2.0 to 3.33 gm moles per hour per kg charge of calcium cyanamide during the simultaneous passage of hydrogen sulphide.
In another embodiment of the present invention the rate of addition of hydrogen sulphide may range between 2.6 to 3.4 gm moles per hour per kg. charge of calcium cyanamide.
In another embodiment of the present invention, the time of addition of 85-90% of total charge of calcium cyanamide may range between 1.8 to 2.2 hour.
In another embodiment of the present invention the time of addition of 10-15% of total charge of calcium cyanamide may range btween xc2xd to 1 hour.
In another embodiment of the present invention the reaction mixture after the addition of 15% of total charge may be kept under a small feed of carbon dioxide ranging from 0.32 to 1.0 gm mole per hour per kg charge of calcium cyanamide.
In another embodiment of the present invention the reaction mass may be retained for a period ranging from 2 to 5 hours after the addition of calcium cyanamide is complete.
In yet another embodiment of the present invention, the rate of addition of carbon dioxide for decomposition of Ca(SH)2 may range from 0.8 to 2.5 gm moles per hour per kg charge of calcium cyanamide.
In a feature of the present invention the calcium carbonate is removed by conventional methods like filtration and the product is recovered by treating the filtrate with activated carbon at 60 to 80xc2x0 C., and subsequent evaporation under reduced pressure and crystallisation at 10-15xc2x0 C. The pressure employed may range from 55 to 65 mm Hg at a temperature of 50 to 70xc2x0 C.
The crystals are filtered and washed with water at 0 to 5xc2x0 C. and dried at 50-80xc2x0 C. The washings of crystals can be recycled to evaporator.
The conversion of thiourea based on calcium cyanamide is 85-92.5% and the yields of the product range from 65-75% with purities ranging 95-98% based on a single crop. Selectivity based on hydrogen sulphide fed is 72-72.5% and 100% on hydrogen sulphide consumed in all cases.
In another feature of the present invention the process can be used even with mother liquors containing thiourea obtained after crystallising it. The quality of the product is comparable with that obtained from the customary process once through.
The invention is illustrated by the following examples, which may not be construed to limit the scope of this invention in any manner whatsoever.